a/ \( (2x+5)(2-x)=4\\↔2x^2-x+10-4=0\\↔-2x^2-x+6=0\\Δ=(-1)^2-4.(-2).6=49>0\\\to Pt\,\, co\,\, 2\,\, nghiem\,\, phan\,\, biet\\x_1=\dfrac{1+\sqrt{49}}{2.(-2)}=-2\\x_2=\dfrac{1-\sqrt{49}}{2.(-2)}=1,5\)
b/ \(\begin{cases}x+3y=11\\3x-y=9-2y\end{cases}\\↔\begin{cases}x+3y=11\\3x+y=9\end{cases}\\↔\begin{cases}3x+9y=33\\3x+y=9\end{cases}\\↔\begin{cases}3x+y+8y=33\\3x+y=9\end{cases}\\↔\begin{cases}8y=24\\x=\dfrac{9-y}{3}\end{cases}\\↔\begin{cases}y=3\\x=2\end{cases}\\Vay\,\, (x,y)=(2;3)\)
